Question

In which of the scenarios can you reverse the dependent and independent variables while keeping the interpretation of the slope meaningful?
As the number of cigarettes smoked in a day increases, the risk for lung cancer increases.
As the age of an infant increases, the height of the infant increases.
As the number of vehicles in a city increases, the amount of pollution in the city increases.
As the concentration of salt in water increases, the density of water increases.
As the number of calls made by a cell phone increases, the cell phone bill amount increases.